A known rope guide for an electric hoist having a drum provided with a helical groove on its peripheral surface comprises a nut whose thread has the same profile and pitch as the groove in the drum of the hoist. The nut consists of three steel shells filled internally with a lining, joined together with the exception of the adjacent ends the first and third sectors which are connected together by a cleat provided with slot through which the rope passes into the rope guide. This slot serves to guide the rope as it wound onto, or unwound from, the drum. At one of its ends the nut has an internal peripheral groove in which the spring pressing member is received. The spring pressing member is tightly pressed against two adjacent rope turns lying in the groove of the hoisting drum. The rope-guiding slot is located between the nut thread and the spring pressing member and is disposed to coincide exactly with the drum groove. When the hoisting drum rotates, the nut does not rotate around its axis, but is moved along the drum axis, pushing the spring pressing member with it. The latter, of course, rotates with the drum and pushed axially under the action of the nut, jumps over the next adjacent turns of the rope.
The above-described known rope guide has the following disadvantages:
It is expensive to manufacture, the machining of the internal thread in the nut sectors being particularly costly. It has considerable deadweight, which decreases the capacity of the hoist. The efficiency of the hoist is reduced by the high sliding friction force, which is the sum of three forces due to friction between the steel drum and the nut thread, friction between spring pressing member and the nut, and friction between the rope and spring pressing member when the latter moves axially along the drum. Furthermore, the rope guide has a short service life, due to the large radial dimension of the rope guide and the fact that the side walls of the slot in the nut, through which the rope passes, are perpendicular to the drum axis. Because of these two factors, when loaded rope deviates from a direction substantially vertical to the drum axis, a large bending moment acts on the radially outer edge of the slot, which bending moment can result in breakage of the nut sectors, or rapid wear of the nut slot and/or of the rope. Finally, the large axial length of the rope guide (amounting to about 5 pitch length of the drum groove) means that the hoisting drum has to be made considerably longer than that required to provide a given hoisting travel of the rope, and the rope wound onto the drum is considerably longer than that required to provide a given height of the load elevation.
Thus, there exists a need for light durable rope guide mechanism with a high efficiency, low cost, and short axial length.